This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-252861 filed on Aug. 23, 2001.
The present invention relates to a method for wet etching and a wet etching apparatus for the method. The method and the apparatus are especially preferably used for manufacturing a diaphragm-type semiconductor sensor chip.
As shown in FIGS. 4A and 4B, a diaphragm-type semiconductor sensor chip 2 such as a pressure sensor has a recess 2a, and a thin film layer 1a, which functions as a diaphragm and is located at the bottom of the recess 2a. The sensor chip 2 also includes components such as a sensing element for measuring a physical quantity, transistors making up a circuit for processing an electric signal from the sensing element, and electrodes for electric connection, which are located on the lower surface of the thin film layer 1a in the vertical direction of FIG. 4B.
In a proposed manufacturing process for the diaphragm-type semiconductor sensor chip 2, a plurality of semiconductor sensor chips 2 are formed in a semiconductor wafer 1, as shown in FIGS. 3A and 3B, and the wafer 1 is divided into the semiconductor sensor chips 2, as shown in FIGS. 4A and 4B. The wafer 1 has an active surface and a back surface. The two surfaces face in the opposite direction. The thin film layer 1a and components are located on the active surface. The recess 2a is formed by selectively etching the wafer 1 using an etchant such as potassium hydroxide aqueous solution from the back surface, which is masked by a mask 3 shown in FIG. 4B. The geometric dimensions of the diaphragm are determined by the size of the recess 2a. 
As shown in FIG. 5, a proposed etching apparatus 11 includes a base member 12, a gasket 13, a vacuum chuck cylinder 14, an etching bath cylinder 15, and a lid 16. The base member 12 is roughly in the shape of a disk. For etching the wafer 1, the wafer 1 is placed on the base member 12 such that the back surface faces upward, as viewed in FIG. 5. The gasket 13, which is attached to the inner surface of the etching bath cylinder 15, seals the periphery of the back surface of the wafer 1. The etching bath cylinder 15 is tightly fixed to the base member 12 by vacuuming the inner space of the vacuum chuck cylinder 14. The wafer 1 is clamped between a projection of the base member 12 and the gasket 13 at the periphery of the wafer 1, as viewed in FIG. 5. An etching chamber 17, which is defined by the wafer 1 as a bottom, the gasket 13 and the etching bath cylinder 15 as a sidewall, and the lid 16 as a lid, is formed, as shown in FIG. 5.
The back surface of the wafer 1 is etched by the etchant when the etchant is supplied into the etching chamber 17. The lid 16 includes an etchant supply port 16a, a rinsing water supply port 16b, and a drain 16c. Deionized water (DIW) is used as the rinsing water. The lid 16 also includes a stirrer 18 for agitating the etchant, a heater 19 for heating the etchant, and an electrode 20 for stopping the etching electrochemically.
However, in the etching using the proposed etching apparatus 11, if there is a defect in the thin film layer 1a, as shown in FIG. 6, the etchant leaks out of the etching chamber 17 through the defect when the etchant reaches the thin film layer 1a. The leaking etchant etches the sensing element, the transistors, and the electrodes on the thin film layer 1a. Even in the case that a wafer is thinned by the etching using the proposed etching apparatus 11, if there is a crack in a thinned part of the wafer, the etchant leaks out of the etching chamber 17 through the crack and the active surface of the wafer is undesirably etched.
The present invention has been made in view of the above aspects with a first object to provide a method for wet etching, with which an etchant is prevented from leaking out of an etching chamber through a defect or a crack generated in a wafer. A second object is to provide a wet etching apparatus for the method. A third object is to provide a method for manufacturing a diaphragm-type semiconductor sensor chip using the method for wet etching and the wet etching apparatus.
In the present invention, a wafer, a first surface of which is protected by a protective film, is mounted on a base member such that the first surface faces the base member. An etching bath cylinder, to which a gasket for sealing the periphery of the wafer on a second surface that is opposite to the first surface is attached, is placed on the wafer. An etching chamber is formed by vacuum chucking with a vacuum chuck cylinder in an etching pot. Nitrogen gas is supplied from a high pressure gas supply source to a hermetic room, which is formed by the base member and the wafer, while being regulated by a pressure regulator. The pressure regulator includes a water reservoir, a decompressing room having an orifice, a first balance tube, and a second balance tube. The wafer is etched while a pressure higher than that applied to the second surface from an etchant is put on the protective film by the nitrogen gas. Therefore, the etchant is prevented from intruding into the hermetic room because the active surface is protected by the protective film and the protective film is supported by nitrogen gas in the hermetic room.